#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>

struct ListNode 
{
    int val;
    struct ListNode* next;
};

struct ListNode* reverseList(struct ListNode* head) {
    struct ListNode* n1, * n2, * n3;
    if (head == NULL || head->next == NULL)
        return head;
    n1 = NULL; n2 = head; n3 = head->next;
    while (n2 != NULL)
    {
        n2->next = n1;
        n1 = n2;
        n2 = n3;
        if (n3)
            n3 = n3->next;
    }
    return n1;
}